Method and apparatus to conduct fluidization of cohesive solids by pulsating vapor flow

ABSTRACT

Cohesive solids are fluidized in a chamber having a movable perforated wall through which the fluidizing gas flows by reciprocating the movable wall.

This is a division of application Ser. No. 07/330,330, filed 3/29/89 nowU.S. Pat. No. 4,939,850.

BACKGROUND OF THE INVENTION

Fluidization of finely divided solids forms the basis of many importantindustrial processes, the best known of which is fluid catalyticcracking. Fluidization seems to work best for solids in a particle sizerange from roughly 5 to 500 microns. There are, however, solids whichlie within this particle size range which stick or cling together andcannot be reliably fluidized. These solids display a property known ascohesiveness. Neat cement is an example of such a material, althoughthere are many other cohesive powders, such as grain dust, finelydivided plastics, and more.

Fluidization of finely divided solids depends on the formation of gasbubbles which pass freely upwardly through a dense phase of particlestotally supported by up-flowing vapor. Cohesive solids tend to form intomasses which are bypassed by the vapor flow through cracks or fissures,rather than through bubbles. These fissures tend to become long-lived,so that the cohesive masses which develop are not subject to mixing. Inmany cases (for example, with neat cement), the solids fluidize verywell for a short time after they are subjected to mechanical stirring.Once the cohesive masses form, however, it may be impossible to breakthem up by gas flow alone.

A well fluidized vessel can be used for blending solids which wouldotherwise need to be accomplished by repeated transfers of the solidsfrom one vessel to another. Also, it is much easier to obtain a steady,controllable flow of solids from a fluidized vessel than from anonfluidized vessel. Clearly, method and apparatus for fluidizingcohesive solids would be very desirable.

STATEMENT OF THE INVENTION

According to one embodiment of the invention, there is provided anapparatus for fluidizing cohesive solids which comprises a vessel havingan upper end and a lower end. A gas permeable partition is positioned inthe vessel between the upper end and the lower end. The partitiondivides the inside of the vessel into a first chamber and a secondchamber. At least a portion of the gas permeable partition is movablealong an axis drawn normal to the partition from a first longitudinalposition to a second longitudinal position. The vessel has an inlet inthe first chamber for the introduction of gas, an inlet in the secondchamber for the introduction of finely divided solids, and an outlet inthe second chamber for exhausting a fluidized mixture of finely dividedsolids and gas which has entered the second chamber through thepartition.

In use, a flow of gas sufficient to fluidize the solids in the secondchamber is introduced into the first chamber. The gas flows through thepartition and fluidizes the solids. The partition is moved back andforth between the first position and the second position to break apartor prevent the formation of cohesive masses in the second chamber.

In a preferred embodiment of the invention, the gas is introduced intothe first chamber at a first flow rate sufficient to move the partitionfrom the first position to the second position. The flow rate is reducedto a second flow rate to permit return of the partition to the firstposition by gravity or other biasing means. By pulsing the gas into thefirst chamber at the first flow rate from time to time, the partitioncan be moved with sufficient frequency to assure reliable fluidizationof cohesive powders in the second chamber.

Bins with "live bottoms" have been used in the prior art to handlecertain kinds of solids. By "live bottom" is meant a bin-bottom whichcan be mechanically vibrated or moved. The inventive device uses a livebottom which is also a gas distribution grid. The movable gasdistribution grid (for example, a perforated plate or a fabric pad, or acombination of both) when the invention is used can easily be made tomove up and down by a pulsating source of fluidizing gas. Cohesivemasses which come in contact with this grid are subjected to mechanicalforces which tend to break them up. In addition, the entire bedalternately expands and shrinks as the gas flow rises and falls, whichprovides another source of mechanical energy to break apart or preventthe cohesive masses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a fluidization system employing certainfeatures of the present invention.

FIGS. 2 through 5 schematically illustrate variations on a portion ofthe device shown in FIG. 1.

FIG. 6 is a cross sectional view of a portion of the device shown inFIG. 5 as would appear when viewed along the indicated lines.

FIG. 7 schematically illustrates another embodiment of the inventionwhere a plurality of perforated partitions are positioned in the samevessel.

FIG. 8 is a cross sectional view of a portion of the device as shown inFIG. 7 as would appear when viewed along the indicated lines.

FIG. 9 is a side elevation, partially sectioned, of a flat-side storagebin constructed in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment of the invention, a vessel 2 is provided which has anupper end 4, a lower end 6 and a longitudinal axis which extends alongthe line indicated by numeral 8 between the upper end and the lower end.At least one gas permeable partition 10 is positioned in the vessel 2between the upper end 4 and the lower end 6 and divides the vessel 2into a first, here lower, chamber 14 and a second, here upper, chamber12. The partition 10 is movable from a first position 16 to a secondposition 18 along a line drawn normal to the partition, in this casecoinciding with the longitudinal axis of the vessel. An annular bracket20 is mounted to an inside surface of the vessel 2 in the illustratedembodiment to define the first position 16 upon which the partitionrests. An annular bracket 22 is positioned on an inside surface of thevessel 2 to form a stop and define the second position 18 to limitmovement of the partition 10 in the upper direction. The vessel 2 has atleast one inlet 24 in the lower chamber 14 for the introduction of afluid, preferably a gas. The upper chamber 12 has at least one inlet 26for the introduction of finely divided solids and at least one outlet 28for exhausting a mixture of the fluid introduced via inlet 24 and thefinely divided solids introduced via inlet 26.

In a preferred embodiment of the invention, the inlet 26 is connected toa source 30 of the particulate material desired to be fluidized. In theillustrated embodiment of the invention, the source 30 is formed by abin 32 which is connected to the inlet 26 by a line 34. A feeder 36 suchas star valve 38 is positioned in the line 34 to feed particulatematerial from the source 30 into the chamber 12 at a desired rate.

A line 40 connects the inlet 24 in the lower chamber 14 with a source 42of fluid, preferably gas, at elevated pressure. Compressor 44 providesthe source 42 in the illustrated embodiment of the invention. The line40 preferably contains a valve 46 positioned between the compressor 44and the inlet 24. By opening and closing the valve 46, the flow of gasfrom compressor 44 into the chamber 14 can be controlled as desired. Asurge tank 48 is preferably positioned between the valve 46 and thecompressor 44 in the illustrated embodiment. A valve 50, preferably acheck valve, is positioned between the surge tank 48 and the compressor44.

In one embodiment of the invention, the chamber 14 is provided with ameans 52 for providing a continuous flow of fluid thereinto. In theillustrated embodiment, the means 52 is formed by manifolds 54 in thelower chamber 14 having fluid outlets establishing pathways from insidethe manifolds to outside the manifolds and a compressor 56 connected tothe inside of manifolds 54 by a line 58 to provide for the continuousgas flow. If desired, a check valve (not shown) can be positioned in theline 58 between the compressor 56 and the manifolds 54 to assist inpressurizing the lower chamber in the hereinafter described manner. Theflow from compressor 56 is preferably sufficient to fluidize the solidparticles introduced into the upper chamber 12 from bin 32, but not sogreat as to cause the partition 10 to be positioned in the secondposition. In operation, gas flow from compressor 56 fluidizes the solidswhile gas flow from compressor 44 is released from time to time from thesurge tank (accumulator) 48 by the actuation of valve 46. Preferably,the valve 46 is of the fast acting type so that the surge tank 48 can beemptied over a period of time on the order of one second or less. Theadditional gas flow from the compressor 44 is sufficient to momentarilyurge the partition 10 into the second position 18 and break up anycohesive masses which may be forming in the second chamber 12. A mixtureof gas from the compressors 44 and 56 and particles from the bin 32 iswithdrawn from the upper chamber 12 through outlet 28 and conveyed tofurther processing by line 60.

A valve 46 which can be used with good results is constructed asfollows: The valve body is formed from a 3 inch pipe nipple having endcaps. A one inch port empties into the nipple from a surge tank throughone end cap; a valve stem having a rubber plug on the end to seal theone inch port enters the nipple through the other end cap. The outer endof the valve stem is spring biased toward the one inch port. A pair ofopposed one inch exhaust posts open through the side wall of the pipenipple. A disk having a diameter of nearly three inches is adjustablymounted on the stem between the opposed exhaust ports and the rubberplug when the plug seals the ports. When pressure in the surge tankdislodges the plug, the high pressure acting on the disk throws thevalve into the wide open position, the disk being moved into alongitudinal position between the opposed exhaust ports and the end capcontaining the valve stem, until the pressure acting on the disk dropssufficiently to permit the rubber plug to reseal the port by action ofthe biasing means. The cycle rate can vary as desired, for example, inthe range of 0.1 to 10 seconds.

There are a number of valving arrangements which can be used to converta continuous source of fluidizing gas to a pulsating source. These canbe considered in two groups: (1) flow interrupting devices which dostore employing surge tank 48, and (2) flow interrupting devices whichdo not store compressed gas as illustrated in FIGS. 7 and 8 for example.The former technique offers the most effective mechanical action, sincethe abrupt release of compressed gas can result in violent mixing.However, the second method as illustrated in FIGS. 7 and 8 does notrequire a source of high pressure gas and may offer sufficientmechanical action for some applications.

In FIG. 7, a manifold 100 delivers fluidizing gas beneath a number ofseparate bin bottom members or grids 102, 104 and 106. Each of the gridscomprises a perforated plate or sheet operatively connected to arestraining member 108 such as by cloth strips 110. Chambers are formedbetween the plates and the support member 108 as illustrated by numerals112, 114 and 116 beneath plates 102, 104 and 106 respectively. Thechambers have fluid inlets 122, 124 and 126 respectively which areconnected to a source 130 of compressed gas such as by the manifoldarrangement 100. Valve 132 is positioned between the inlet 122 and thesource 130. Valve 134 is positioned between the inlet 124 and the source130. Valve 136 is positioned between the inlet 126 and the source 130.Flow of gas to each of the chambers 112, 114 and 116 is controlled bythe valves 132, 134 and 136 which are in the normally closed positioned.Flow controlling valves 132, 134 and 136 could be pneumatically actuatedball valves, for example. The total vapor flow, which would ordinarilybe delivered to all grids at once, is delivered to one grid at a time ina preferred embodiment of the invention. The superficial gas velocitythrough the grid is sufficient to lift the grid off of the support 108.When the flow is interrupted, the receiving grid 102, 104 and 106 fallsback to the support 108 and another grid is picked up by the vapor flow.

The system illustrated by FIGS. 7 and 8 is best applicable to solidsthat are not extremely difficult to fluidize and can be carried outwithout a source of high pressure gas. The system illustrated in FIG. 1,however, is preferred for difficult to fluidize solids because thesudden release of pressure mechanically shocks the bed of solids to anextent determined by the amount of pressure used.

FIGS. 2 through 5 show variations of the plate 10 of FIG. 1 which can beused in accordance with the invention.

In FIG. 2, a vessel 200 is provided with a perforated plate 202circumferentially connected by a fabric 204 to an inside circumferenceof the vessel 200. The largest dimension of the plate 202 is smallcompared with the diameter of the vessel where the plate is positioned,such as in range of from 10% to 50% of the diameter of the vessel.

In FIG. 3, a plate 302 is connected to the inside wall of a vessel 300by fabric 304. The arrangement in FIG. 3 is similar to that shown inFIG. 2 except that the plate 302 is larger, on the order of 75% to 95%of the diameter of the vessel. The fabric which circumferentiallyconnects the plate with the interior of the vessel can be lighter inweight than the fabric used in the device of FIG. 2 since the structuralrequirements are less.

FIG. 4 shows an embodiment of the invention which can be used in hightemperature applications although it is not limited to such uses. Aplate 402 rests on an inside flange 406 which extends circumferentiallyaround an inside surface 410 of a vessel 400. A rim 412 extends axiallyfrom an outside circumference of the plate 402 in a direction away fromflange 406. The rim 412 is received by an annularly shaped baffle 414mounted circumferentially around the inside surface 410 of the vessel400. The baffle 414 defines the second position for the perforated plate402 and prevents excessive fluid channeling around the circumference ofthe baffle. Support members such as I beams 416 can be provided crossingan inside surface of the vessel 400 if structurally required.

In FIG. 5, a perforated flexible metal sheet 502 forms a partitionacross an inside surface of a vessel 500. The metal sheet 502 is fixedlyattached to the inner surface such as by welding to an annular flange504. In the first or lower position (not shown), the flexible sheet 502can rest against suitable support members such as I beams 516.

With reference to FIG. 6, apertures 520 through the partition willgenerally be quite small, since the particles to be fluidized generallyrange in size from 5 to about 500 microns, frequently between 20 and 200microns. Those skilled in the art can readily calculate the gas velocityrequired to prevent particle migration downwardly through the aperturesand gas velocity across the cross section of the second chamber in orderthat the particles stay fluidized. Generally speaking, the total area ofthe apertures 520 will be less than 20% of the cross sectional area ofthe vessel. The distance between the first position for the partitionand the second position for the partition can vary over a wide range.Generally, based on vessel diameter, the positions will be separated byfrom about 2% to about 50% of the vessel diameter, usually from about 5%to about 25% of a vessel diameter. The thickness of the flexible metalsheet will vary depending upon vessel diameter. Generally however, thesheet will have a thickness in the range of from about ten thousandthsto one hundred thousandths of an inch.

Referring to FIG. 9, there is shown a partial side elevation of a dryparticulate material bulk storage bin having parts of the housing brokenaway, generally designated by the numeral 910. The bin 910 isparticularly adapted for storing particulate flowable materials such asdry cement, flour and other relatively fine powdered materials. The bin910 includes a depending portion 912 having sloping flat sidewalls 914and 916 which converge toward an outlet trough portion 918. The troughportion 918 may include an auger or screw type conveyor disposed thereinfor displacing material from the bin. An interior chamber 920 formedwithin the bin 910 is delimited in part by the sidewalls 914 and 916 andthese walls are each fitted with at least one fluidizing pad generallydesignated by the numeral 924. Each of the pads 924 is preferablyconstructed of a somewhat porous canvas like material and is suitablysecured to the interior surface of the walls 914 and 916, respectively,around its perimeter. The pads 924 are responsive to the injection ofpressure air into a narrow space 926 formed between each pad and theinterior wall surface of the walls 914 and 916 to flex the pad and todisseminate pressure air into the chamber 920 to assist in fluidizing orcausing the material stored therein to flow when such action is wanted.The operation of the pads 924 is enhanced by the injection of arelatively high pressure pulse or blast of pressure air into the spaces926 by way of respective conduits or manifolds 928 disposed on theexterior surfaces of the walls 924 and 926, respectively. The manifolds28 are adapted to include one or more pressure air discharge pipesconnected thereto and suitably connected to the walls 914 and 916 fordischarging pressure air through suitable openings in the walls into thespaces 926.

Pressure air is supplied for operating the fluidizing pads 924 by way ofrespective sources including reservoir tanks 932 which may receivepressure air from a common source such as a compressor 934 by way of asuitable supply conduit 936. The tanks 932 each are connected to aninlet conduit 938 of a self controlling air blast or pulse type controlvalve 940. The control valves 940 are each connected to a manifold 928by way of a suitable conduit 942. In one mode of operation of fluidizingmaterial in the chamber 920 repeated pulses or blasts of pressure airare emitted into the chambers 926 by way of the reservoir tanks 932 andthe valves 940. A control valve 944 may be suitable remotely controlledto provide pressure air to the tanks 932. With the supply of pressureair to the tanks 932 the pressure in each tank increases until therespective control valves 940 self actuate to deliver a pulse or blastof pressure air through the respective manifolds 928 into the spaces926. By providing the bin 910 with a plurality of self actuating valves940 feeding from a common source, sufficient fluidizing action can beobtained without an elaborate control system.

In each of the described embodiments there exists the possibility ofparticles going into the first chamber, by going either around orthrough the partition. Where this problem is presented, aparticle-receiving boot can be provided in the first chamber, with aport for removing the accumulated particles. If desired, the particlesremoval port can be connected with a riser for automatic return of theparticles to the second chamber.

A properly designed gas distribution grid offers sufficient flowresistance so that the gas will flow evenly through all parts of thegrid. The movable grids need to have a flow resistance such that thehighest gas flow will create a back pressure under the grid sufficientto lift the grid along with a layer of unfluidized solids which may beresting on it. Canvas pads are already commonly used as grids in cementbins, although not with pulsating gas flow. These could be convertedinto pulsating grids by the addition of flow interruption apparatus inthe source of fluidizing air. The addition of a perforated plate underthe fabric pad offers the advantage of stiffening the pad particularlyat the spot directly opposite to the gas inlet, and also enables thedesigner to tailor the flow resistance to the expected gas flow. Glasscloth or wire reinforced gas cloth may be substituted for canvas forgreater strength.

While various preferred embodiments of the invention have been shown anddescribed herein, the invention herein is not to be so limited, exceptto the extent such limitations are found in the claims.

What is claimed is:
 1. Apparatus comprising:a vessel having an upper end and a lower end; and a gas permeable partition positioned in the vessel between the upper end and the lower end, said partition dividing the vessel into a first chamber and a second chamber, at least a portion of the gas permeable partition being movable along an axis drawn normal to the plane of the partition between a first longitudinal position and a second longitudinal position, said vessel having an inlet in the first chamber for the introduction of a fluid, an inlet in the second chamber for the introduction of a finely divided solid, and an outlet in the second chamber for exhausting a mixture of the fluid and the finely divided solid from the second chamber; and a pulsating source of fluidizing gas to said first chamber.
 2. Apparatus in claim 1 wherein said pulsating source of fluidizing gas comprises a gas compressor and a flow interrupting device operably connected to the inlet in the first chamber.
 3. Apparatus as in claim 2 wherein said gas permeable partition comprises a perforated metal plate.
 4. Apparatus as in claim 2 wherein said gas-permeable partition comprises a fabric connector connecting said gas permeable partition to said vessel.
 5. Apparatus as in claim 2 further comprising a surge vessel positioned between the gas compressor and the inlet into the first chamber and a gas flow control valve activated by gas pressure positioned between the surge vessel and the first chamber. 